One mistreats Jack and his employer? (6)
I believe the answer is:
abuser
'one mistreats' is the definition.
'jack and his employer?' is the wordplay.
'jack' becomes 'ab'.
'and' is a charade indicator (letters next to each other).
'his employer?' becomes 'user'.
'ab'+'user'='ABUSER'
(Other definitions for abuser that I've seen before include "He takes undue advantage" , "One who reviles or a misuser of drugs" , "Bully" , "one who uses bad language" , "Person who over-indulges in, say, drugs or drink" .)